DEDHAM, Maine — Why a Massachusetts man crashed head-on into a pickup truck on U.S. Route 1A a month ago may never be known, Maine State Police Sgt. Alden Bustard said Wednesday of the fatal crash.

“I think our investigation has gone as far as it can go,” the sergeant said. “We haven’t been able to determine what caused him to cross the centerline.”

Dallas Whatley, 39, of Taunton, Mass., was killed in the Dec. 20 collision when his car crossed the centerline of Route 1A near the Dedham Town Office and hit the pickup. He was visiting friends in the area and was driving a 2004 Infiniti north at around 9:40 a.m. when he struck a 2006 Chevrolet pickup driven by James Sullivan of Sorrento. Whatley died at the scene and Sorrento suffered a leg injury.

The investigation, which is mostly complete but pending toxicology test results, has determined that Whatley was not using his cellphone to send or receive text messages during the fatal crash, Bustard said.

“We don’t have any reason to believe that he was [texting],” the sergeant said. “We don’t have any reason to believe that alcohol was involved.”

Once the toxicology tests return, likely in the next few weeks, the crash investigation will be closed, Bustard said.